In recent years, a liquid crystal alignment film obtained by aligning a liquid crystal polymer or a liquid crystal compound having a polymerizable functional group has been developed as an optical film (e.g., optical compensator) used for liquid crystal displays. Such a liquid crystal alignment film has attracted attention since it is possible to implement a high-degree alignment state (e.g., tilted alignment or twist alignment) that cannot be implemented by a birefringence film that utilizes polymer film stretching technology.
A cholesteric polarizer that utilizes the selective reflectivity of a liquid crystal alignment film (selective reflection film) obtained by subjecting a composition that includes a liquid crystal polymer or a polymerizable liquid crystal compound (e.g., (meth)acrylate compound) and a chiral compound to a cholesteric alignment process has also been put to practical use.
The selective reflection center wavelength λ is indicated by “λ=n×P” (where, n is the average refractive index, and P is the cholesteric pitch). The selective reflection wavelength band Δλ is indicated by “Δλ=Δn×P” (where, Δn is (ne−no) (where, ne is the extraordinary refractive index, and no is the ordinary refractive index). Therefore, a material having a large value Δn (i.e., high optical anisotropy) is required to widen the selective reflection wavelength band Δλ.
When using a selective reflection film for a liquid crystal display as a cholesteric polarizer, the selective reflection film must selectively reflect light in the visible region. Since the selective reflection wavelength band Δλ of a single selective reflection film is normally narrower than the visible region, a plurality of selective reflection films are laminated to widen the selective reflection wavelength band Δλ. Specifically, the number of layers of a selective reflection film increases when using a material having a narrow selective reflection wavelength band Δλ. As a result, productivity decreases. Therefore, a material (e.g., polymerizable liquid crystal compound) having a large value Δn (i.e., a wide selective reflection wavelength band Δλ) has been desired.
However, since a known polymerizable compound or the like that has a large value Δn exhibits poor solubility, applicability, and alignment properties, it may be impossible to produce a uniform film, or it may be difficult to obtain a selective reflection film that exhibits practical alignment properties.
An azine shown by the following formula (A) has been known as a liquid crystal compound.
wherein Ra represents an alkyl group, and Rb represents an alkyl group, a cyano group, a fluorine atom, a trifluoromethoxy group, or the like.
The above azine compound is a liquid crystalline material that shows a liquid crystal phase over a wide temperature range, is relatively chemically stable, and can be produced inexpensively, for example.
However, the above azine compound does not necessarily exhibit satisfactory mutual solubility with a liquid crystal compound that are widely used at present. The mutual solubility of the azine compound can be improved to some extent by increasing the number of carbon atoms of the side-chain alkyl group in the formula (A). However, the resulting azine compound shows a liquid crystal phase in a narrow temperature range.
In order to solve the above problem, Patent Document 1 proposes a liquid crystal compound shown by the following formula (B).
wherein Rc represents a hydrogen atom or an alkyl group having 1 to 12 carbon atoms (the double bond has a trans configuration when Rc represents an alkyl group), p represents an integer from 1 to 10, q represents 0 or 1, W, X, and Y represent a fluorine atom, a chlorine atom, a methyl group, a cyano group, or a hydrogen atom, and Z represents a fluorine atom, a chlorine atom, a cyano group, an alkyl group or an alkoxy group having 1 to 12 carbon atoms, or an alkenyl group or an alkenyloxy group having 3 to 12 carbon atoms, provided that one or more hydrogen atoms included in these groups may be replaced by a fluorine atom.
The above compound is chemically stable against heat, light, and the like, has excellent liquid crystallinity, and can be easily produced industrially. Since the above compound has excellent mutual solubility with a liquid crystal compound or a liquid crystal composition, the liquid crystal response time can be significantly improved by utilizing the above compound. Therefore, the above compound is considered to be useful as a component of a liquid crystal material for a liquid crystal display element that shows a liquid crystal phase over a wide temperature range and has a quick response time.
However, since an improvement in performance of liquid crystal displays has been increasingly desired, development of a liquid crystal material that shows a liquid crystal phase over a wider temperature range, is chemically stable, can be inexpensively produced, and has a large value Δn has been desired.
Patent Document 1: JP-A-10-147562